Impact of Native Environment in Multiheme-Cytochrome Chains of the MtrCAB Complex.

MtrCAB protein complex plays a crucial role in exporting electrons through the outer membrane (OM) to external acceptors. This complex consists of three proteins and contains 20 hemes. Optimal protein-protein interactions and, consequently, heme-heme interactions facilitate efficient electron transfer through the conduit of hemes. The cytochrome MtrA remains mostly inside porin MtrB, and the MtrB barrel contains two calcium ions on its surface. In this study, we investigate the effect of porin-bound calcium ions on the heme-heme distances in the twenty-heme network. We performed all-atom molecular dynamics simulations of the OM-protein complex, MtrCAB, in the presence and absence of the MtrB-bound calcium ions. We observe that the calcium ions bound to MtrB affect the interfacial heme-heme distance when all of the hemes are oxidized and impact one of the heme-heme distances in MtrC when all of the hemes are reduced. In both cases, the absence of calcium ions increases the heme-heme distance, highlighting the crucial role of calcium ions in maintaining the heme network, which is essential for long-range charge transport.